From the Guidelines
Typical values for serum uric acid and urine sodium in a patient with SIADH who is clinically euvolemic are a serum uric acid concentration of < 4 mg/dL and a urinary sodium level of > 40 mEq/L.
Key Laboratory Findings
- Serum uric acid concentration is typically low, with values < 4 mg/dL, due to the increased uric acid clearance associated with SIADH 1.
- Urine sodium level is inappropriately high, with values > 40 mEq/L, indicating that the kidneys are retaining too much water and not enough sodium, leading to hyponatremia 1.
Diagnostic Criteria
- SIADH is characterized by euvolemic hypoosmolar hyponatremia, with low serum osmolality and inappropriately high urine osmolality in the absence of diuretic treatment, adrenal insufficiency, heart failure, cirrhosis, and hypothyroidism 1.
- The diagnosis of SIADH can be supported by assessing the effective arterial blood volume with the fractional excretion of urate, which can approach 95% accuracy 1.
From the Research
Typical Values for Serum Uric Acid and Urine Sodium in SIADH
- Serum uric acid levels are typically low in patients with Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) 2
- Urine sodium concentration is usually high in SIADH, with values often above 30 mEq/L 3, 4, 2
- A urine sodium concentration of 50 mEq/L can be used to separate SIADH from hypovolemic hyponatremia, with a sensitivity of 0.89 and specificity of 0.69 4
- Fractional excretion of uric acid (FE-UA) is a useful marker for diagnosing SIADH, with a cutoff value of 12% providing a positive predictive value of 100% 3, 5
Clinical Euvolemia in SIADH
- Patients with SIADH are typically euvolemic, with normal or slightly elevated blood volume 3, 2
- Clinical euvolemia is characterized by the absence of signs of volume depletion, such as orthostatic hypotension or tachycardia 2
- In euvolemic patients with SIADH, urine sodium concentration is often high, reflecting the inappropriate secretion of antidiuretic hormone 3, 4, 2
Laboratory Parameters in SIADH
- Serum uric acid levels are often low in SIADH, while urine uric acid levels are high 2, 5
- Fractional excretion of sodium (FE-Na) is also elevated in SIADH, reflecting the inappropriate secretion of antidiuretic hormone 2, 5
- The combination of high urine sodium concentration, low serum uric acid levels, and elevated FE-UA can be used to diagnose SIADH in clinically euvolemic patients 3, 4, 2, 5